This invention relates to the precise locating of a structure and, more particularly, to such precise locating using holographic techniques.
Structures such as optical devices must often be angularly located (that is, established) relative to related structures in an optical train. The required locating may involve highly precise establishing of all or some of the various angular degrees of freedom of the structures relative to each other. Such alignments are required for both testing of the optical devices and also in their final service applications.
In one approach used to achieve highly precise locating, a series of optical targets in the form of mirrors or prisms are mounted on the structure to be aligned. Light beams are directed against individual ones of the series of optical targets, the reflected or refracted light beams are received at a sensor, and angular information is determined from the received light beams. The structure is angularly repositioned based upon the measurements, and the measurements may be repeated as necessary. The angular locating by this technique may be made precise to the limits possible with the mounting and optical techniques.
While operable, this approach has several drawbacks. The precise mounting and aligning of several optical targets is often time consuming. The optical targets must typically be located within an aperture viewing area of the measuring device. For some situations, this poses no problem. In others, the aperture viewing area is quite small, and it is physically difficult or impossible to locate all of the required optical targets within the aperture viewing area.
There is a need for an improved approach to the precise locating of structures such as optical devices in optical systems. The improved approach is desirably compatible with small-scale structures where all of the locating structure must be within a small aperture viewing area. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method for angularly locating a structure relative to a reference surface. The present approach achieves highly precise locating, to optical-quality standards. It is compatible with small, compact systems wherein the aperture viewing area available for accomplishing the locating is small. This technique allows locating to be accomplished to a good standard of precision with the naked eye, or to much higher standards using instrumentation. When instrumentation such as an interferometer is used, the same instrumentation may be utilized when appropriate to measure the quality of the structure being located.
In accordance with the invention, a method for angularly locating a structure relative to a reference surface comprises the steps of providing the structure to be angularly located relative to the reference surface, providing an optical target having a reflective angular-reference hologram thereon with an effective angular orientation, and affixing the optical target to the structure. An alignment light beam is directed against the angular-reference hologram on the optical target, and a signal return of the alignment light beam is received at a measurement location of a measurement device. The angular orientation of the structure is adjusted to vary the signal return to correspond to a locating of the alignment light beam relative to the reference surface.
The angular-reference hologram may be made with an image of a reflective surface such as a mirror, either photographically or synthetically, at an effective angular orientation using well-known techniques. When the angular-reference hologram is viewed, there is a strong reflection from the angular-reference hologram mirror surface only when the angular-reference hologram is viewed from exactly the angular orientation that is perpendicular to the effective angular orientation of the holographic mirror surface. At other angular orientations, there will be little if any reflection. The optical target, and thence the structure to which it is attached, is located to the desired angular orientation by directing a light beam toward the angular-reference hologram (which is affixed to the structure to be located), monitoring the return signal from the angular-reference hologram, and adjusting the angular orientation of the optical target until the maximum intensity of return is observed.
In one preferred embodiment, there are two or more reflective angular-reference holograms, with different effective angular orientations, on the optical target. These multiple reflective angular-reference holograms may be spatially superimposed upon each other, or spatially separated on the optical target. Maximum returns are observed at each angular orientation defined by one of the angular-reference holograms, allowing the optical target and structure to be angularly located with great precision at any of a number of selected discrete angular orientations using the one optical target.
The angular-reference holograms may be quite small in size. The use of small angular-reference holograms and/or spatial superposition of larger angular-reference holograms allows the optical target to be quite small yet fully effective in achieving the locating of the optical target and thence the structure. Even a complex optical target may therefore be accommodated in a small area and within a small aperture viewing area. The single angular-reference hologram or at least one of the multiple angular-reference holograms is desirably oriented to define a reflective perpendicular to the angular-reference hologram and thence to the optical target, and the adjusting step achieves a parallel orientation of the optical target to the reference surface.
The present approach has the important advantage that it may be used with alignment apparatus of varying degrees of sophistication. A quickly obtained, reasonably good approximation of the desired orientation may be achieved with the naked eye. More sophisticated measurement apparatus may be used to improve the precision of angular locating, and wavefront analysis equipment such as an interferometer may be used to locate the optical target to exact angular orientations.
The angular-reference holographic optical target is inexpensive to produce either photographically or virtually with computer-generated holograms. Holographic optical targets may be mass produced, applied to apparatus, and left in place to allow re-alignment at a later time. The optical targets are readily applied to a flat surface, as with an adhesive backing, reducing the time involved for angular alignment as compared with prior approaches which required affixing mirrors or prisms to the structure to be located. The holographic optical targets may instead be removed after use, and later re-applied to the same structure or applied to different structure. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.